The most important element of the system is the map. The guidance systems only need to direct the computer to the accuracy of one road junction before the map takes over. The GPS locates the car roughly within 25 metres. From here, it can work out the car's position on the map. If you drive out of the area covered by the map, the arrow merely points directly towards your destination. Once you are on the map, the software looks you up.

    The GPS isn't used very often. The compass works out which way the car is facing, so it can calculate in which direction you are travelling along the road.

    The system keeps track of how far the car has journeyed down the road by measuring the distance travelled. The normal cable-driven odometer is not accurate enough for this purpose so the system uses the magnets in the anti-lock braking system to measure the revolution of the wheels. This needs careful calibration but can still be fooled. Tyre wear can fool the system - but only until you make the next turn. When this happens, the computer checks to see where you are likely to be - it's only going to be a few yards out - and resets the position. The result of all this is a car which always knows where to go.